We are investigating the role of nucleotides in regulation of liver growth. Evidence from our own and other laboratories has suggested possible participation of certain cyclic nucleotides, highly phosphorylated nucleotides, and perhaps also diadenosine tetraphosphate. As cell cultures offer advantages over whole animal experimentation, we have initially established a system for studying adult rat hepatic parenchymal cells cultured as monolayers in a serum free medium, virtually free of contamination with other cell types. These short term cultures respond to stimulation of 3H-thymidine incorporation into DNA by epidermal growth factor (EGF), enhanced by insulin. We are employing high pressure liquid chromatography to explore levels and turnover rates of the various nucleotides in stimulated and unstimulated hepatocyte cultures in parallel to similar studies in livers stimulated by partial hepatectomy or hormonal or dietary manipulation in vivo.